In general, in order to manufacture an as-cast strip (which is a general term for a slab, a billet, a bloom, a beam blank, and the like) by a continuous casting process, molten steel is supplied from a ladle, and then passes through a tundish for storing the molten steel, a submerged nozzle, and a mold. The molten steel is then cooled in a mold by cooling effect thereof, and forms a solidified shell. The solidified shell formed by cooling the molten steel is completely solidified into an as-cast strip by second cooling water that is injected out of the spray nozzles, while being guided by guide rolls disposed under the solidified shell.
During a continuous casting process of steel, when the molten steel is provided into the mold, an additional substance such as mold flux is also added into the mold. Mold flux is generally provided into the mold in a solid state, such as powder or granules, and melted by the heat generated by the molten steel supplied in the mold to control the heat transfer between the molten steel and the mold and improve lubrication.
As shown in FIG. 1, mold flux provided into the mold as granules is melted on the surface of the molten steel 12 and sequentially forms a liquid layer 21, a sintered layer (semi-solid layer) 23, and a powder layer 25 in this order from the melt-surface. The liquid layer 21 easily transmits radiation waves having wavelengths between 500 nm and 4000 nm emitted from the molten steel because it is substantially transparent. The sintered layer 23 and the powder layer 25, however, are optically opaque, so that they prevent a rapid drop in temperature of the melt-surface by blocking the radiation waves.
However, in the related art, after the mold flux in the form of powder or granules is melted by the heat generated by the molten steel, the liquid layer 21 flows between the mold 10 and the solidified shell 11, and then solidified onto the inside wall of the mold 10 to form a solid slag film 27, while a liquid slag film is formed on the molten steel side. Accordingly, the heat transfer between the molten steel and the mold can be controlled and lubrication property is improved.
In this case, the mold flux, which is attached to the mold at a position where the molten slag inflows between the solid slag film 27 and the solidified shell 11, protrudes toward the inside of the mold 10. The mold flux protruding toward the inside of the mold is called a slag bear 29. The slag bear 29 prevents molten slag from flowing between the mold flux film 27 and the solidified shell 11.
The amount of mold flux consumption per unit area of an as-cast strip is suppressed by the slag bear 29. In general, the faster the casting speed increases, the more the mold flux decreases; therefore, lubrication efficiency between the as-cast strip and the mold is decreased and break-out is caused. In addition, since the thickness of liquid mold flux becomes irregular due to the slag bear 29, the shape of the solidified shell 11 becomes irregular in the mold 10 and surface cracks are developed, which gets worse as the casting speed increases.
Korean Unexamined Patent Application Publication No. 1998-038065 and U.S. Pat. No. 5,577,545 disclose methods of restricting the growth of a slag bear by applying graphite or fine carbon black to decrease the melting speed of the mold flux. However, these methods cannot basically prevent a slag bear. In addition, non-uniformity occurs during solidification because non-melted mold flux inflows between the solidified shell and the mold when the melting speed of the mold flux is slow. As a result, the break-out becomes worse.
Methods of injecting mold flux to the melt-surface after melting outside are disclosed in Japanese Unexamined Patent Application Publication Nos. 1989-202349, 1993-023802, 1993-146855, 1994-007907, 1994-007908, 1994-047511, 1994-079419, 1994-154977, and 1994-226111. However, all of the documents above propose restrictively using molten mold flux in an early state of the casting and using powder-typed mold flux after the casting reaches a normal state. Accordingly, it is difficult to maintain the temperature of the surface of molten steel by the methods, since the molten mold flux is substantially transparent for wavelengths between 500 and 4000 nm as described above, so that radiation waves emitted from the molten steel easily pass through the mold flux resulting in increase of the radiation heat transfer. For this reason, after a predetermined time passes in the casting process, the surface of the molten steel is solidified. Therefore, the continuous casting process cannot be smoothly performed.
Further, paper was used to supply molten mold flux into the mold, but it has limitation in supplying molten mold flux throughout the continuous casting process.